I. Field of the Invention
The invention relates to improvements in the catalytic hydrocracking of hydrocarbonaceous feedstocks to produce lower boiling hydrocarbon products. In particular, it relates to an improved process for suppressing the normal tendency in hydrocracking hydrocarbon feedstocks, which have a propensity to form polynuclear aromatic hydrocarbon compounds (PNA's) during hydrocracking, to form compounds which foul the hydrocracking unit.
II. Backgrounds and Problems
In a typical hydrocracking process a heavy product stream which contains unconverted feed molecules and the PNA's formed during the process, is recycled back to the conversion reactors through a heat exchanger zone. In this zone the recycle stream is cooled which can lead to precipitation of a portion of the relatively insoluble PNA's. The precipitated PNA's have a tendency to form a crust on the interior surface of the heat exchangers thereby reducing heat transfer capabilities, this limiting throughput and interfering with the performance of the unit. Ultimately, this can lead to plugging of the unit and forced shutdown. Removal of the PNA's in the recycle stream can improve process flexibility, increase unit cycle length, and probably improve product quality. This problem is recognized, and addressed in, e.g., U.S. Pat. No. 4,447,315. Other solutions to the problem are nonetheless needed.